memtest: Support arch-specific and EFI memtest86+ 6.00

• BIOS: Final filename doesn't change (memtest), but now uses a 64-bit version on amd64.
• EFI: Now included as memtest.efi; 32-bit on i386, 64-bit on amd64. As it's GRUB-only, we can test for the existence of memtest.efi (6.00+ only) in GRUB itself.
• Add @MEMTEST_DIR@ to facilitate directory-based testing in GRUB.

This expands upon the previous MVP commit 15e78a3e .